(1) Field of the Invention
The present invention relates to processing of a substrate such as a semiconductor wafer, and more particularly relates to a dummy substrate used for cleaning and drying and to a semiconductor processing method using the dummy substrate.
(2) Description of Related Art
Basic processes for fabricating a semiconductor integrated circuit device include a cleaning process and a drying process of the semiconductor wafer. A cleaning method using a basin has been widely used in a cleaning process step. In this method, a semiconductor substrate is soaked into a chemical solution which contains acid or alkali and with which a chemical-resistant basin is filled, thereby removing contaminants on the surface of the semiconductor substrate.
A chemical-solution-circulation-type cleaning apparatus with a basin cleans each of lots of semiconductor substrates (for example, a set of twenty-five semiconductor substrates that will be fabricated together) by circulating, through a pump and a filter, a chemical solution filling in a basin. This apparatus has a problem in saving the chemical solution because a large volume of the solution is used for cleaning. Furthermore, since the cleaning by the apparatus is a batch treatment, there also exists another problem that the states of cleaned surfaces of semiconductor substrates are varied in each production lot.
In order to solve the above problems, drum-type spin cleaning apparatuses using rotating drums have been developed. Such an apparatus requires only a small volume of chemical solution for cleaning and provides stable cleaning so that cleaning is rapidly completed and a solution is not left after cleaning. This can reduce variations in the states of cleaned surfaces of semiconductor substrates in a lot. Therefore, drum-type spin cleaning apparatuses are now used frequently (see, for example, Japanese Unexamined Patent Publication No. 2002-52358).
A drum-type spin cleaning apparatus, a known art, will be described hereinafter with reference to the drawings.
FIG. 5 is a diagram illustrating an example of an internal structure of a drum-type spin cleaning apparatus when viewed from side.
As shown in FIG. 5, a turntable 11 is placed inside a cylindrical chamber 10, and a shaft 12 is coupled to the lower surface of the turntable 11 at the site of a rotation axis of the turntable 11 and furthermore is connected to a motor 13 located below the chamber 10. The rotation of the motor 13 allows the turntable 11 to rotate through the shaft 12 at a high speed of, for example, 1000 rpm or more.
Bar guides 15 are placed on the turntable 11 to fix cassettes 14 into which a plurality of substrates including a plurality of to-be-processed substrates 16, such as semiconductor substrates, and at least one dummy substrate 17 are inserted. The use of the guides 15 allows the cassettes 14 to be placed symmetrically about the rotation axis of the turntable 11. The arrangement of the cassettes 14 and guides 15 will be described later in detail with reference to FIG. 6.
The dummy substrate 17 represents herein a substrate on which an electronic device for providing an actual product is not formed, for example, a silicon substrate or the like on which an oxide film or a silicon nitride film is formed.
A spray nozzle 19 protrudes from the middle part of a top lid 18 of the cleaning apparatus to be perpendicular to the top lid 18. The closure of the top lid 18 allows the spray nozzle 19 to be located above the rotation axis of the turntable 11. During cleaning, a chemical solution for cleaning 20 containing acid or alkali can be horizontally sprayed from the spray nozzle 19 onto the to-be-processed substrates 16 or the dummy substrate 17. The chemical solution 20 is supplied from the outside of the cleaning apparatus to the inside of the cleaning apparatus through a chemical solution supply pipe 21.
Cleaning is carried out by spraying the chemical solution 20 onto the to-be-processed substrates 16 and the dummy substrate 17 and rotating the cassettes 14 together with the turntable 11. This makes uniform cleaning possible.
Not only a chemical solution but also pure water can be sprayed by the spray nozzle 19. At least one side spray nozzle 22 for supplying additional pure water during the cleaning is also mounted in the vicinity of and with a certain distance from the turntable 11. Pure water is supplied through a pure water supply pipe 23.
FIG. 6 is a diagram illustrating an example of the internal structure of the drum-type spin cleaning apparatus when viewed from above.
FIG. 6 illustrates a turntable 11, cassettes 14 placed on the turntable 11, guides 15 through which the cassettes 14 are fixed, a plurality of substrates including a plurality of to-be-processed substrates 16 and at least one dummy substrate 17 and put into each cassette 14. The other components are not shown. As illustrated in FIG. 6, four sets of (eight) guides 15 are placed on the turntable 11, and the cassette 14 is fixed by the associated sets of guides 15. These guides 15 allow two or four cassettes 14 to be placed symmetrically about the rotation axis.
The dummy substrate 17 is often used for cleaning or drying process by the above-mentioned drum-type spin cleaning apparatus. The dummy substrate 17 is used mainly for two purposes.
One of the purposes is to prevent particles from being deposited on the to-be-processed substrates 16.
As seen from FIG. 5, the cassettes 14 are longitudinally fixed in the chamber 10 of the drum-type spin cleaning apparatus. Spin drying is carried out immediately after cleaning. During this drying, some of particles in the chamber 10 are raised with the rotation of the turntable 11 and floats in the chamber 10. Such particles are deposited on the to-be-processed substrates 16 or the like put into the cassettes 14, leading to reduced production yields. Particles are likely to be deposited, in particular, to the uppermost one of the plurality of to-be-processed substrates 16 put into each cassette 14.
To cope with this, not the to-be-processed substrate 16 on which an electronic device is formed but the dummy substrate 17 is placed in the uppermost part of each cassette 14. This allows particles to be deposited almost intensively on the dummy substrate 17. Therefore, particles are much less likely to be deposited on the to-be-processed substrates 16. This can prevent reduction in yields.
The other purpose is to achieve balanced rotation.
As long as the gross mass of a combination of each cassette 14 and the plurality of substrates including the to-be-processed substrates 16 and the dummy substrate 17 and contained in the cassette 14 (hereinafter, referred to as “cassette gross mass”) does not vary from cassette to cassette, the cassettes 14 are placed on the turntable 11 symmetrically about the rotation axis of the turntable 11. This makes balanced rotation possible. However, if at least one of the plurality of cassettes 14 has a different cassette gross mass from that of each of the other cassettes, this makes balanced rotation impossible. To avoid this, the cassette gross masses of all the cassettes 14 are allowed to coincide with one another by adding a necessary number of dummy substrates 17 to the cassette 14 having a different cassette gross mass. Dummy substrates 17 are used for this purpose.
This will be described below in detail by citing a specific example.
Four cassettes 14 can be provided for the drum-type spin cleaning apparatus shown in FIG. 6. Therefore, if the number of the to-be-processed substrates 16 to be cleaned or dried at the same time is a multiple of 4, the same number of the to-be-processed substrates 16 can be contained in each cassette 14. In this case, each cassette 16 has the same cassette gross mass. Therefore, the masses of the cassettes 14 are balanced, resulting in balanced rotation kept.
However, if the number of the to-be-processed substrates 16 to be processed at the same time is not a multiple of 4, more to-be-processed substrates 16 will be contained in at least one of the cassettes 14 than in each of the other cassettes 14. As a result, the at least one of the cassettes 14 has a different cassette gross mass from that of each of the other cassettes 14. Thus, if cleaning and drying are carried out by rotating the cassettes 14, eccentric rotation is caused.
Furthermore, only two cassettes 14 may be used for the drum-type spin cleaning apparatus shown in FIG. 6. In this case, if the number of the to-be-processed substrates 16 to be processed at the same time is odd, eccentric rotation is caused likewise.
If the turntable 11 is allowed to rapidly spin with such eccentricity, the drum-type spin cleaning apparatus itself vibrates due to centrifugal force. The increase of the vibrations of the drum-type spin cleaning apparatus might cause the drum-type spin cleaning apparatus itself, the to-be-processed substrates 16 and the dummy substrate 17 to be broken.
The drum-type spin cleaning apparatus usually has an interlock. Thus, if eccentricity is caused, the apparatus will stop. However, in order to perform cleaning, electric rotation itself must be prevented. Therefore, each cassette 14 need have the same cassette gross mass. A necessary number of dummy substrates 17 are added to the cassette 14 having less to-be-processed substrates 16 than the other cassettes 14. This allows the total number of substrates in any one cassette 14 including the to-be-processed substrates 16 and the at least one dummy substrate 17 (hereinafter, referred to as “the total number of contained substrates”) to be the same as that for any other cassette 14. This provides balanced mass, because the mass of the to-be-processed substrate 16 is substantially the same as that of the dummy substrate 17.
Lots often have different numbers of the to-be-processed substrates 16. Therefore, the number of the dummy substrate to be used is adjusted for each lot, thereby balancing rotation.
An electronic device fabricating process includes 100 or more individual process steps. Substrates on which electronic devices are not formed are used also in process steps other than the above-mentioned cleaning and drying process steps. Such substrates on which electronic devices are not formed are referred to as “dummy substrates” herein. For example, the dummy substrates include substrates used to determine electronic device fabrication conditions for each process step and dummy substrates used to stabilize conditions on which an electronic device is fabricated in a dry etching process step and a CVD process step.
More dummy substrates are required with increase in the production of semiconductor devices or the like. This leads to increased cost. Therefore, inexpensive dummy substrates that can repeatedly be used have been desired (see, for example, Japanese Unexamined Patent Publication No. 2000-272910).
Conventional dummy substrates have the following problems.
The conventional dummy substrates used in a semiconductor fabricating process include a silicon substrate, a silicon substrate whose top surface is covered with an insulating film, such as a silicon oxide film or a silicon nitride film, and the like.
The dummy substrates repeatedly used for semiconductor fabricating process steps are highly chemical-resistant. In spite of this, in a cleaning process step using a high-temperature chemical solution with etching ability, such as a mixed solution of NH4OH and H2O2, the dummy substrate is etched little by little with increase in the number of electronic device fabricating processes, leading to the decreased thickness of the dummy substrate. Since a mechanical strength of the dummy substrate is consequently deteriorated, the dummy substrate that has been used for a certain period must be exchanged.
If the dummy substrate with the deteriorated strength keeps being used, this increases the possibility that it will be broken in the cleaning apparatus during process steps requiring high-speed rotation. The breakage of the dummy substrate will produce broken pieces and particles of the dummy substrate. The produced broken pieces and particles contaminate substrates on which electronic device products are formed and which are processed in the same batch, leading to the reduced yields. Furthermore, they also contaminate the chamber of the drum-type spin cleaning apparatus.
Such a state provides the necessity for cleaning the inside of the chamber to recover the particle level to the extent that an integrated circuit can be fabricated. Since this cleaning is nevertheless very difficult, the chamber itself is usually not cleaned but exchanged. It is expensive to repair a drum-type spin cleaning apparatus in this manner, and the spin cleaning apparatus cannot be operated during repair.
As seen from the above, the known method using dummy substrates has a problem that production cost becomes high.